The σ- and π-bonding modes of a tetraanionic porphyrinogen ligand in zirconium(IV) complexes: a theoretical investigation

Abstract

The bonding modes of the porphyrinogen ligand L^4–(H₄L = 5,10,15,20,22,24-hexahydroporphyrin) with the metal ion Zr⁴⁺ are analysed in detail for the limiting co-ordinations η⁵,σ,η⁵,σ and σ,σ,σ,σ. It is concluded that in both co-ordination modes σ bonding is by far the most dominant, mainly due to strong charge donation from the pyrroli nitrogen lone pairs into the empty 4d_x²-y² and 4d_xz, with additional effects from donation into the 4d_z² and 5s orbitals. The π bond, resulting from donation from occupied pyrrolyl π orbitals into the metal 4d_xy and 4d_yz orbitals is significant for η⁵,σ,η⁵,σ- but rather weak for σ,σ,σ,σ-co-ordination due to the diminished donation into the 4d_xy orbital. The total orbital interaction contribution (the covalent component) is about one third of the ionic component of the bond, the latter being identified as the sum of the Pauli repulsion and the attractive electrostatic interaction between L^4– and Zr⁴⁺. The ionic contribution is about the same in the two configurations, but the covalent component of the bond decreases by ca. 3 eV for σ:σ:σ:σ co-ordination due mostly to a weaker π-bond interaction. The η⁵,σ,η⁵,σ complex is only 2.1 eV more stable, indicating that the interconversion between the two co-ordination modes is a relatively easy process. The most stable co-ordination mode is preserved after interaction of the substrate with the Lewis bases tetrahydrofuran or H^–.